Several new animal model systems have been developed to evaluate the efficacy of immunoconjugates. A human melanoma (FeMX) and a human colon (Ht-29) xenografted either in nude mice or rats were found to induce a progressively growing tumors at the site of inoculation (sc) or induce pulmonary foci (iv). A T-cell lymphoma has been adopted to grow in nude mice following the sc inoculation of 4 million cells. Specific MoAb's exist for each of these animal model systems. The administration of 50 Upsilon g's of a gelonin or pokeweed antiviral protein (PAP) conjugate of 9.2.27 administered iv to nude mice bearing pre-palpable FeMX tumors demonstrated a 3 to 4 day delay in tumor emergence as compared to untreated animals. However, there was no difference in median survival time (MST) between the two groups when the study was terminated at 60 days. Localication studies have revealed that 1 to 2% of I125-labeled 9.2.27 localizes in FeMX tumors (7-8 mm) while 2 to 5% of the labeled MoAb was found in liver, kidney and spleen. Only .05% or less of a gelonin or PAP conjugate of I125-labeled 9.2.27 (50 Upsilon g) reached the site of similar size FeMX tumors. Two to 5% of the conjugates were taken up by the liver, kidney and spleen. Pretreatment of tumor-bearing animals with a non-specific MoAb (RPC-5) followed by systematic administration of I125-labeled 9.2.27 failed to enhance the uptake of the labeled MoAB to the tumor site or decrease uptake of the specific MoAb ty the reticuloendothelial system (RES).